Noxides

Noxides, shorthand for nitrogen oxides (NOx), are a group of reactive nitrogen-oxygen compounds that primarily consist of nitric oxide (NO) and nitrogen dioxide (NO2). In some contexts, related species such as N2O3, N2O4, and N2O5 are also discussed, especially in atmospheric chemistry, but NO and NO2 are the most significant for most practical and environmental considerations.

Properties and chemistry: NO is a colorless diatomic radical gas, while NO2 is a reddish-brown oxidant gas. Both arise from oxidation of nitrogen and readily participate in further reactions in air and aqueous media. NO can be oxidized to NO2 in the atmosphere, and NOx species act as key intermediates in the formation of nitric acid and nitrates. They are highly reactive and can participate in photochemical smog formation, ozone generation, and acid deposition.

Occurrence and sources: NOx are produced during high-temperature combustion, including motor vehicles, power plants, and industrial processes. Natural sources include lightning and certain microbial processes. In industry, NOx are generated intentionally as intermediates in the production of nitric acid via the Ostwald process, where ammonia is catalytically oxidized to NO and NO2 before becoming HNO3.

Uses and regulation: NOx serve as reagents and precursors in chemical synthesis and fertilizer production. They are also regulated emissions due to environmental and health concerns; control technologies such as selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) are employed to minimize NOx emissions from engines and industrial plants. Monitoring and reporting of NOx emissions are common in air quality management programs.

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